Measuring light properly: lux, DLI, and the phone app that lies
Almost every succulent problem is a light problem, and almost nobody measures it. A 25-dollar lux meter turns guesswork into arithmetic — provided you know the three ways the measurement goes wrong.
Lux versus DLI, and which one to use
Lux is instantaneous illuminance, weighted to the human eye. DLI — daily light integral — is the total photosynthetic photon dose over a day, expressed in mol/m²/day. For succulents the useful DLI range is roughly 25–45 mol/m²/day; below 15 they stretch, above 60 they burn without acclimatisation.
Converting: for a white LED, DLI ≈ lux × hours × 0.0000245. So 30,000 lux for 13 hours gives about 9.6 mol/m²/day, which is well short of the target. Getting to 30 mol/m²/day needs either 40,000 lux for 12 hours plus a sunny window, or 60,000 lux for 14 hours. This single calculation explains why so many indoor collections stretch under lights that look bright.
Measurement error one: measuring at the wrong plane
Reading at the pot rim overstates the dose at the leaves by 20–40 % because the canopy is closer to the fixture. Read at the top of the rosette, and read at three points across the shelf — the centre reading under a bar light is typically 30 % higher than the ends.
Also check the distance decay. A bare LED bar at 30 cm gives roughly 4× the lux it gives at 60 cm. If your shelf is 60 cm deep and the light is a single bar down the middle, the plants at the front and back are getting a quarter of what the centre gets.
Measurement error two: trusting a phone app
Phone camera lux apps are unreliable above about 15,000 lux; the sensor saturates and the app extrapolates. In a side-by-side test across three phones in the same position, readings at what was actually 35,000 lux ranged from 12,000 to 48,000 — an error of nearly half to double.
They are usable for one thing: relative comparison. If the app says 9,000 in one place and 4,500 in another, the ratio is probably roughly right even though the absolute figure is not. Use it to find the brightest spot, then use a real meter to set the height.
Measurement error three: forgetting the window factor
Window glass cuts 15–25 % of visible light, and a window screen cuts another 30–50 %. A double-glazed south window with a bug screen therefore delivers roughly half of what the outdoor reading suggests. Measure on the inside face of the glass, at the height of the plant, at midday — not by holding the meter outside.
Season matters as much. A window that gives 50,000 lux in June gives 6,000 in December at the same latitude, a factor of eight, driven mostly by sun angle rather than day length. That is the number to plan supplementary lighting around.
| Month | State | What the plant wants |
|---|---|---|
| January | Growth | Deepest rest. Water only if the lowest leaf ring softens; keep the room as bright as you can. |
| February | Growth | Late rest. Daylight is returning — begin adding an hour of supplementary light every two weeks. |
| March | Growth | Repotting window opens for summer growers. Trim roots, refresh the mineral fraction, dry for a week. |
| April | Growth | Growth restarts. First full watering at the rim, then resume the published interval. |
| May | Growth | Peak growth and the offset season. Feed at quarter strength every third watering. |
| June | Growth | Lengthening days, rising heat. Watch the leaf turgor test rather than the calendar. |
| July | Growth | Hottest weeks. Water in the evening, a third of the spring volume, and only on a visible symptom. |
| August | Growth | Heat eases at night. Prepare mix and pots for the autumn repotting window. |
| September | Growth | Autumn window for winter growers. Root-prune and repot while the soil is still warm. |
| October | Growth | Peak autumn growth. This is the season when most colour work should be done. |
| November | Growth | Daylight falls fast. Taper watering by a quarter and stop feeding entirely. |
| December | Growth | Slow, dry and bright. This is the colour peak for most genera in the collection. |
Tinted rows are the months this genus is genuinely out of action: no repotting, no feeding, and water only if the plant visibly shrinks.
Four things to take away
- Aim for 25–45 mol/m²/day DLI; 30,000 lux for 13 hours is only about 9.6.
- Read at the top of the rosette and at three points across the shelf, not at the rim.
- Phone lux apps are unusable above 15,000 lux except for relative comparison.
- Double glazing plus a bug screen halves the light; plan for the December figure, not June.
Informational only — This page is a cultivation record, not horticultural or plant-pathology advice. Any treatment should follow the product label and local regulation.